Four phase stepping motor control

ABSTRACT

This invention describes an improved system for controlling a stepping motor for use in driving a perforated paper tape in a tape reading apparatus. The motor is provided with eight poles and corresponding coils which are connected in pairs of diametrically opposite coils. These four sets of coils are driven by a power amplifier that is controlled by two flip-flop circuits. Arrangements are made so that successive identical pulses on an input line will switch one flip-flop then the other then the first, and so on. By this means the magnetic flux in a stepping motor is shifted by 45* of rotation by each successive pulse. The motor has a six pole rotor, and as the flux is shifted by 45* in one direction, the rotor rotates 15* in the opposite direction. Thus, on the application of successive pulses on the input, the rotor advances by 15* in a continuing direction.

Davenport FOUR PHASE STEPPING MOTOR CONTROL [75] Inventor: Raymond A.Davenport, Okemah,

Okla.

[73.] Assignee: Oktronics, Inc., Okemah, Okla.

[22] Filed: Aug. 30, 1972 [21] Appl. N0.: 284,723

[52] U.S. C1 318/696, 310/49, 318/685 [51] Int. Cl. H02k 37/00 [58]Field of Search 318/696, 685, 674, 663; 310/49 [56] 7 References CitedUNITED'STATES PATENTS 3,660,746 5/1972 Milek 318/696 3,381,193 4/1968Smith 318/696 3,385,984 5/1968 ORegan.. 318/696 3,445,741 5/1969 Gerber318/696 3,005,941 10/1961 Heggen.... 318/696 3,250,977 5/1966 Heggen....310/49 X 3,239,738 3/1966 Welch 310/49 Apr. 2, 1974 [57] ABSTRACT Thisinvention describes an improved system for controlling a stepping motorfor use in driving a perforated paper tape in a tape reading apparatus.The motor is provided with eight poles and corresponding coils which areconnected in pairs of diametrically opposite coils. These four sets ofcoils are driven by a power amplifier that is controlled by twoflip-flop circuits. Arrangements are made so that successive identicalpulses on an input line will switch one flip-flop then the other thenthe first, and so on. By this means the magnetic flux in a steppingmotor is shifted by 45 of rotation by each successive pulse. The motorhas a six pole rotor, and as the flux is shifted by 45 in one direction,the rotor rotates 15 in the opposite direction. Thus, on the applicationof successive pulses on the input, the rotor advances by 15 in acontinuing direction.

2 Claims, 6 Drawing Figures FOUR PHASE STEPPING MOTOR CONTROL CROSSREFERENCES TO RELATED APPLICATIONS This application is related to twocopending applications, Ser. No. 284,768 entitled: Tape Drive D. C.

Motor Control System, filed Aug. 30, 1972, and Ser. No. 284,779entitled: Light Potentiometer Drive D. C. Motor Control System, filedAug. 30, 1972.

BACKGROUND OF THE INVENTION This invention is in the field of datahandling systems. More particularly it is related to perforated papertape reading devices. Still more particularly it concerns the means fordriving a stepping motor for reliably advancing a perforated paper tapein such reading devices.

In the prior art there are numerous devices which utilize theperforations in a strip of paper for recording and reading data. Thedata are in the form of perforations in rows transverse to the length ofthe strip. Generally the paper strip. has a center column of smallerperforations which are used by a sprocket means driven by a paper drivemotor, which advances the strip in incremental movements equal to thespacing between each sprocket hole. In general the strip is advanced onespace at a time by a pulse on an input circuit, which advances the drivemotor by an incremental angle, such as to drive the paper by thedimension of the spacing between the two sprocket holes. Since the rowsof data perforations are aligned with the sprocket holes, each step ofthe motor will advance a new set of perforations into the readingdevice, and so on.

It is a primary object of this invention to provide a simplifiedelectronic system, responsive to a series of identical impulses on aninput line, to advance a drive motor, or stepping motor, by equalincrements of angle so as to advance the perforated tape by equalincrements of distance equal to the spacing between the sprocketperforations.

.This and other objects are realized and the limitations of the priorartare overcome in the improved apparatus of this invention.

SUMMARY OF THE INVENTION This invention is concerned'with theincremental movement of a prerecorded perforated paper tape through areading apparatus so that with successive identical impulses on theinput connection the perforated tape will be advanced incrementally bythe spacings between perforations, and thus between rows of dataperforations. The improvement lies in the drive circuit of the motorwhich drives the sprocket which drives the tape.

The motor is a commercial device which comprises a stator field havingeight salient poles with corresponding windings. The windings areconnected in pairs, each coil being connected to the diametricallyopposite coil. When one pair of coils is powered there will be adiametrical field between the two opposite poles. When two adjacentpairs of poles are simultaneously excited, there will be a diametricalflux, of greater total flux, which will be centered in the gap betweenthe two sets of poles.

If a rotor having aneven number of salient poles such as six for exampleis placed in the motor then a pairof rotor poles will lineupdiametrically with the direction of the flux set up by the field poles.By exciting a third set of coils and dropping the first set of coils,the flux will be shifted by the spacing of one pole, namely 45. Therotor will then move correspondingly to align the nearest salient poleto the new flux direction. As will be described below, this will cause arotation of an angle of 15. The paper drive sprocket is connected to theshaft of the motor and thus the paper strip is moved by a correspondingdimension, which is equal to the longitudinal spacing of dataperforations.

The four sets of coils of the motor are connected to four poweramplifiers which are driven through four preamplifiers, which are drivenby two sets of flip-flop devices. Each flip-flop has a true andcomplement output such that when voltage appears on the true output, novoltage appears on the complement output, and vice versa. The first andthird set of coils are connected to the true and complement outputs ofthe first flip-flop. The second and fourth sets of coils are connectedto the true and complement outputs of the second flipflop. By a seriesof interlock connections, a first input pulse will operate oneflip-flop. The second pulse will operate the second flip-flop, the thirdpulse the first one, and so on, so that on each successive identicalinput pulse, successive coils in the motor are excited to form anadvancing pair of coils, and an advancing flux which will drive themotor in steps of 15.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of thisinvention and a better understanding of the principles and details ofthe invention will be evident from the following description taken inconjunction with the appended drawings in which:

FIGS. 1 and 2 show elevation and plan views of a typ ical paper drivesystem such as those to which the motor drive system of this inventionwould be applied.

FIG. 3 indicates a generalized tape reading and driving apparatus.

FIG. 4 indicates a schematic diagram of the construction of the drivemotor.

FIG. 5 illustrates how on reconnecting the coils the field can beadvanced and the rotor similarly advanced.

FIG. 6 is a schematic wiring diagram of the drive system for the motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingsand in particular to FIGS. 1 and 2, the numeral 10 indicates generally atypical tape drive system to which this invention can be applied. Thereis a transport deck 12 to which are supported the various pieces ofapparatus required. There are two tape reels 13 and 14 which aresupported on shafts of drive motors 32, 32A, a tape 16 is reeled off ofthe reel- 14 over an idler pulley l7 and over an idler pulley 18 whichis mounted on a dancer arm 19 which is supported in bearings in theplate 12. The dancer arm shaft is connected to a sensor 30 which can beof several different types, such as a potentiometer, for example.

The tape then passes over an idler 21 through the tape reading apparatuswhich will be described in connection with FIG. 3, over a tape drivesprocket 28, around the idler 18A, on a second dancer arm 19A,

over idler 17A, and is woundon to the reel 13. The tape drive motor orstepping motor 29 is mounted'behind the panel 12 and drives the shaft 27which supports the sprocket 28.

Referring now to FIG. 3, the tape 16 passes between elements 24 and 26.The element 22 is a lamp, 24 is an optical system which provides eightsets of focused beams, which pass downward to a corresponding group ofeight photoelectric detectors in the box 26. These beams are spacedapart in a direction perpendicular to the tape 16 to correspond to theprescribed spacing of the eight columns of perforations, which representthe data punched into the tape. In the center of the tape, between theeight columns of perforations, there is a central column of smallerperforations, having equal longitudinal spacing. This center row ofperforations is for the driving and positioning of the tape 16. This isaccomplished by a sprocket shown dashed as elements 28 which is mountedon and driven by the shaft 27. Element 34 is for the purpose of guidingthe paper tape through the reading apparatus. Dashed outline 29corresponds to the outer diameter of the motor which is mounted behindthe panel. The size and dimensions of l the motor sprocket, etc., are sodesigned that when the motor shaft 27 rotates by an angle of 15 thesprockets will rotate a corresponding angle and will drive the paperlongitudinally by a dimensionof l/ inch which is the spacing along thetape of the rows of perforation. Thus each rotation of the shaft ofpositions a new set of perforations under the reading apparatus.

Refer now to FIG. 4. The drive or stepping motor is shown schematicallyas a cylindrical shell 40 with a plurality of inwardly projecting poles42A,42B, 42C 42H. These are spaced apart by an angle of- 45 and have aspace 44 between them,.such that coils can be inserted over the poles.The coils are indicated by the numerals 46A, 46B, 46C 46H. The coilswound around the poles are. connected in pairs with coils wound aroundopposite poles, so that when one is excited they are both excited, andthere will be a diametrical flux between the two poles. Consequently itis possible to number the poles and coils in terms of A, B, C, D, A, B,C, and D, there being four sets or pairs of coils rather than eightcoils. This will require only four sets of amplifiers, one connected toeach of the sets of coils. Each coil has an input lead such as 45A, 45B,45C, 45D, and an input common lead 47.

Supported in the center of the stator in bearings, is a rotor 48 mountedon a shaft 50. This rotor has six radial poles. When a diametrical fluxis set up by a pair, or two pairs, of poles the rotor will turn in sucha way as to align one of its set of colinear radial poles in-line withthe flux, and will be essentially locked into that position. Now whenthe diametrical flux is advanced by an angle of 45 there will be'norotor poles aligned with it. The nearest polewill be one which isl5 awayand therefore being thenearest pole the rotor will turn 15 to'align thispole into the diametrical flux. Thus, for each advance of the field fluxof 45 the rotor will advance by 15. I i

This can be seen more clearly in connection with FIG. 5. The elements46C, 46D, 46A,'46B, shown as rectangles along the line 52 represent thefield poles which are laid out on a, flat planeyThe angle between thesepoles is represented by numerals 0, 15, 30, 45, etc., which representdegrees. In the line 53 are squares represented by numerals 49E, 49F,49A, 49B, etc.

These represent the poles of the rotor. On the assumption that coils 46Cand 46D are excited, (as shown by thecross-hatching), the diametricalfiux will align itself with the zero degree line, and the rotor willpull in so that one pole, such as 49F, is aligned with the direction ofthe flux, the zero degree line. In line 54 the cross hatching shows-thatcoils 46D and 46A are now exited and the diametrical flux will be inalignment with the 45 line, file flux having advanced from the zerodegree to the 45 line. With the flux now at 45, and the rotor poles inaccordance with the position on line 53, it is seen that pole 49A is now15 away from the flux while pole 49F is 45 away from the flux. When thecoils 46D and 46A are excited, the flux transfers to the 45 line, pole49A moves to the left to align itself with the flux,

and the position of the rotor is now shown in line 55, where the pole49A is aligned with the flux. With the poles 46A and 46B excited, asshown in line 56, the flux now moves to 90 angle, the pole 49B is now 15away from the flux and is pulled into alignment as shown in line 57.

Provided the connections can be switched to the coils in accordance withdiagram of FIG. 5, it is seen that when the coils are excited insuccessive pairs, the flux will move to the right, which represents aclockwise rotation of the flux. The rotor will move to the left in acounterclockwise direction, by an angle of 15 for each 45 advanced ofthe flux. What is required now is an electronic system for providing theproper switching of the coils in response to a successive series ofidentical pulses. This will be described in connection with FIG. 6.

FIG. 6 represents schematically the drive system for the stepping mot'or29. The dashed rectangle represents the four sets of coils 46 of themotor 29. The dashed rectangle 68 represents four power amplifierswhichare connected to and drive the four sets of coils of the motor. Thedashed rectangle 69 represents schematically the four preamplifierswhich drive the four power amplifiers 68. The dashed rectangle 62represents the first of the two flip-flops, and rectangle 64 representsthe second of the two flip-flops. Numeral 66 represents the interlocksystem by means of which successive pulses on the input line operate oneflip-flop and then the other, and then the first and so on.

Consider the first flip-flop 62. This has two transistors 72A and 72Band it has two outputs A and 80B. The first output 80A is called a trueoutput and the second, 80B is called a complemenet output. Similarly,the second flip-flop 64 has a true output 80C and a complement output80D.

If the coil pairs are numbered in sequence around the motor there willbe a first pair, a second pair, third pair, fourth pair, first pair,second pair, etc. In operation there are always two pairs excited at thesame time. For instance, the first and second pairs will be excited. Onthe next impulse the first pair is disconnected and the third pair'isexcited, so that the field now' is set up by the second and the thirdpairs. On thenext impulse the second pair is disconnected and the fourthpair is excited so that the field now is set up by the third and fourthpairs. It is seen that the true output 80A of the first flip-flopcontrols the transistor 740 which controls the power amplifier 76C. Thepower amplifier 76C controls the coil pair 46A. The complement output80B of the first flip-flop controls the peramplifier 74A which controlsthe power amplifier 76A which controls the coil pair 46C.

Incidentally while the four coils are shown as being 90 apart they are,as previously described only 45 apart and so the coil 46C is now 90 awayfrom coil 46A. Referring to the second flip-flop 64 the true output 80Cis connected to the preamplifier 74D which drives the power amplifier76D which controls the coil pair 463 and the complement output 80Dcontrols the preamplifier 74B which controls the power amplifier 76Bwhich controls the coil pair 46B.

At the start, consider that the flip-flop 62 true output 80A has avoltage on it and correspondingly the true output 80C of flip-flop 64has a true voltage on it, so that coils 46A and 46B will be excited. Onthe next impulse on input line 81, flip-flop 62 will switch the trueoutput 80A off and switch the complement output 80B on, which will causepower amplifier 76A to provide power to the coil pair 46C and, ofcourse, the true output 80C of the flip-flip 64 is still connected sothat coils 46B and 46C are now excited. Thus, this first switching ofthis first flip-flop has dropped coil 46A and added coil 46C and 46C and46B are excited and the flux has been shifted 45. On the next pulse onthe input line 81, the second flip-flop is switched from the true to the74A to driver transistor 76A cuts off the current through the winding46C so when winding 46A is conducting 46C must have no current, and viceversa. Similarly transistor 72C is cut off permitting current to flowthrough winding 46B while transistor 72D is conducting cutting off thecurrent through winding 46D.

complement output and voltage now is applied to line 80D to preamplifier74B of power amplifier 76B to coil 46D. Meanwhile the true output 463has been disconnected. Power is supplied to the coil pairs 46C and 46Dand the resultant flux is now shifted by another 45". On the next pulsethe flip-flop 62 will then revert back to the true output, and on thesucceeding pulse flip-flop 64 will revert from the complement to thetrue output, and conditions will be the same as at the starting point.

Looking at the circuit in more detail it is seen tha when thetransistors 72A to 72D have a high voltage, this occurs when they arenot conducting. With a voltage on line 80A for instance, or when thetransistor 72A is non-conducting the potential of line 80A rises almostto the potential of the supply voltage on line 82. Thus, the base leadof the transistor 74C is raised, the transistor 74C conducts and thevoltage across the emitter resistor 83C rises, causing the base of thetransistor 76C to rise, and the transistor 76C conducts current throughthe coil 46A from the power supply voltage on lead 82, to ground. Inother words by cutting off transistor 72A the transistor 76C is made toconduct and in a sense, with one'end of the motor coil connected topositive supply voltage on line 82, the transistor connects the otherlead .to ground. This provides essentially full voltage across the coilpair 46C.

This same procedure happens when any of the four transistors areconnected. It is a property, of course, of the flip-flop that only oneor the other of the transistors 72A, 728 for example, can conductsimultaneously. When one starts conductingthe other is automaticallyextinguished. So by connecting the two outputs 80A and 808 to sets ofcoils which are 90 away, a single H operation of the flip-flop 62, forinstance, will cause the first coil to be disconnected and the secondcoil 90 I this voltage applied through emitter follower transistor Inthis condition a trigger pulse applied to the input line 81 goes to atrigger bus 84. This bus is connected to condensers 85A, 85B, 85C and85D. The junction of condenser 85A and diode 86A is held near groundpotential by the collector of conducting transistor 72D which goes byway of lead 87D. The positive-going transition on the pulse on thetrigger bus, differentiated through condenser 85A is blocked by diode86A. However, the negative-going transition drives the cathode of diode86A more negative than the anode. Diode 86A then conducts, driving thebase of transistor 72A more negative. Since transistor 72A is alreadycut off this pulse has no effect.

The invention of condenser 85B and diode 86B is held at a positivepotential by the collector of nonconducting transistor 72C by a lead 87Cso that neither portion of the trigger pulse differentiated throughcondenser 858 can drive the cathode of transistor 728 more negativesthan its anode. Diode 86B therefore does not conduct and a trigger isnot felt on the base of the transistor 72B. Thus the first pulse is noteffective in changing the conduction of transistor 72A or 72B. Similarlya high voltage level at the junction of condenser 85C and diode 86Cisolates the trigger from the base of the transistor 72C but a lowvoltage at the junction of condenser 85D and diode 86D permits anegative spike to reach the base of transistor 72D. Transistor 72D isdriven out of saturation the current through the collector resistor isreduced and the potential applied to the base of transistor 72C.Transistor 72C goes into conduction driving the base of transistor 74Dfurther negative. The result is that transistor 72C is driven intosaturation while transistor 72D is cut off.

In a similar manner additional pulses on the input line 81 will causesuccessive operation of the two flip-flops as has been indicatedpreviously. Successive pulses on input line 81 will cause the operationsindicated previously to cause a clockwise rotation of the motor fieldwhich will cause a counterclockwise rotation of the rotor. A similarconnection 87 for the input pulses will operate the flip-flops in aslightly different order which will then cause a rotation of themagnetic flux to be in a counterclockwise direction and the rotor willrotate in a clockwise direction. This will constitute a reverse rotationof the motor and will cause a reverse movement of the paper strip.

While the invention has been described with a certain degree ofparticularity it is manifest that many changes may be made in thedetails of construction and the arrangement of components. It isunderstood that the invention is not to be limited to the specificembodiments set forth herein by way of exemplifying the invention, butthe invention is to be limited only by the scope of the attached claimor claims, including the full range of equivalency to which each elementor step thereof is entitled.

What is claimed is:

L'In a perforated paper tape reading system includmg:

means to store, drive, reel and unreel the perforated paper tape, andincluding photoelectric means to read the perforations in said tape;said means to drive said tape comprising means to cause linear movementof said tape past said reading means in accordance with the angularrotation of a tape drive shaft; said drive shaft driven by a steppingmotor designed to advance its shaft a selected angular increment ofrotation in response to an electrical signal; said motor having eightpoles spaced at 45 around the stator, and a rotor with six salient polesspaced at 60, each stator pole having a coil, each coil having adiametrically opposed coil with which it is connected, to form fourpairs of coils, the improvement in electrical control means comprising:input control means responsive to identical sequential input-pulses;means responsive to a first pulse to said input control means to providepower to a first and to a second adjacent coil pair 45 displaced fromsaid first coil pair; and means responsive to a second pulse to saidinput control means to disconnect said first coil pair and connect powerto a third adjacent coil pair 45 displaced from said second coil pairand 90 displaced from said first coil pair; said control means includinginterlock means including first flip flop means for sequentiallyapplying power to said first and third coil pairs, and second flip flopmeans for sequentially applying power to said second and fourth coilpairs; whereby the magnetic field set up in a diametrical direction bysaid two coil pairs will rotate by an angle of 45 in response to saidinput pulse, and said "rotor will turn by an angle of in the directionopposite to the rotation of said field. I 2. in a tape drive system inwhich said tape is driven in incremental steps, and including a steppingdrive motor having four pairs of diametrically opposed poles spaced at45 around the stator, each pole having a coil, the eight coils connectedinto four diametrically opposed pairs, the rotor of said motor havingsix salient poles spaced 60 apart, the improvement comprising controlmeans for advancing the diametrical magnetic flux set up by said pairsby an angle of 45 inresponse to a pulse to the input of said controlmeans, comprismg:

a. a first, second, third and fourth amplifier means,

connected respectively to a first, second, third and fourth pair ofcoils;

b. a first and second flip-flop means, each having a true and acomplement output, such that when voltage appears on one output itdisappears from the other;

e. the true output of the first flip-flop connected to the firstamplifier which is connected to the first coil pair;

d. the complementary output of the first flip-flop connected to thethird amplifier which is connected to the third coil pair which isspaced from said first coil pair;

e. the true output of said second flip-flop connected to the secondamplifier which is connected to said second coil pair between, and 45from, said first and third coil pairs;

f. the complement output of said second flip-flop connected to thefourth amplifier which is connected to the fourth pair of coils, whichis 90 from the second pair of coils; and

g. means, including interlock means, which in response to a series ofpulses on the input to the control means will:

1. on the first pulse flip the first flip-flop from true to complement,thus disconnecting power from the first coil pair and connecting powerto the third coil pair; whereby power will be on the second and thirdcoil pairs;

2. on the second pulse flip the second flipflop from true to complement,thus disconnecting the second coil pair and connecting power to thefourth coil pair, whereby power will be on the third and I fourth coilpairs;

3. on the third pulse flop the first flip-flop from the complementary tothe true connection, thus disconnecting power from the third coil pair,and connecting power to said first coil pair, whereby power will be onthe fourth and first coil pairs; and

4. on the fourth pulse flop the second flip-flop from the complementaryto the true, switching power from the fourth'coil pair to the secondcoil pair whereby power will be on the first and second coil pairs;

whereby as each pulse is received the direction of the diametricalmagnetic field set up by the four coil pairs, operating two at a time,will advance 45.

1. In a perforated paper tape reading system including: means to store,drive, reel and unreel the perforated paper tape, and includingphotoelectric means to read the perforations in said tape; said means todrive said tape comprising means to cause linear movement of said tapepast said reading means in accordance with the angular rotation of atape drive shaft; said drive shaft driven by a stepping motor designedto advance its shaft a selected angular increment of rotation inresponse to an electrical signal; said motor having eight poles spacedat 45* around the stator, and a rotor with six salient poles spaced at60*, each stator pole having a coil, each coil having a diametricallyopposed coil with which it is connected, to form four pairs of coils,the improvement in electrical control means comprising: input controlmeans responsive to identical sequential input pulses; means responsiveto a first pulse to said input control means to provide power to a firstand to a second adjacent coil pair 45* displaced from said first coilpair; and means responsive to a second pulse to said input control meansto disconnect said first coil pair and connect power to a third adjacentcoil pair 45* displaced from said second coil pair and 90* displacedfrom said first coil pair; said control means including interlock meansincluding first flip flop means for sequentially applying power to saidfirst and third coil pairs, and second flip flop means for sequentiallyapplying power to said second and fourth coil pairs; whereby themagnetic field set up in a diametrical direction by said two coil pairswill rotate by an angle oF 45* in response to said input pulse, and saidrotor will turn by an angle of 15* in the direction opposite to therotation of said field.
 2. on the second pulse flip the second flip-flopfrom true to complement, thus disconnecting the second coil pair andconnecting power to the fourth coil pair, whereby power will be on thethird and fourth coil pairs;
 2. In a tape drive system in which saidtape is driven in incremental steps, and including a stepping drivemotor having four pairs of diametrically opposed poles spaced at 45*around the stator, each pole having a coil, the eight coils connectedinto four diametrically opposed pairs, the rotor of said motor havingsix salient poles spaced 60* apart, the improvement comprising controlmeans for advancing the diametrical magnetic flux set up by said pairsby an angle of 45* in response to a pulse to the input of said controlmeans, comprising: a. a first, second, third and fourth amplifier means,connected respectively to a first, second, third and fourth pair ofcoils; b. a first and second flip-flop means, each having a true and acomplement output, such that when voltage appears on one output itdisappears from the other; c. the true output of the first flip-flopconnected to the first amplifier which is connected to the first coilpair; d. the complementary output of the first flip-flop connected tothe third amplifier which is connected to the third coil pair which isspaced 90* from said first coil pair; e. the true output of said secondflip-flop connected to the second amplifier which is connected to saidsecond coil pair between, and 45* from, said first and third coil pairs;f. the complement output of said second flip-flop connected to thefourth amplifier which is connected to the fourth pair of coils, whichis 90* from the second pair of coils; and g. means, including interlockmeans, which in response to a series of pulses on the input to thecontrol means will:
 3. on the third pulse flop the first flip-flop fromthe complementary to the true connection, thus disconnecting power fromthe third coil pair, and connecting power to said first coil pair,whereby power will be on the fourth and first coil pairs; and
 4. on thefourth pulse flop the second flip-flop from the complementary to thetrue, switching power from the fourth coil pair to the second coil pairwhereby power will be on the first and second coil pairs; whereby aseach pulse is received the direction of the diametrical magnetic fieldset up by the four coil pairs, operating two at a time, will advance45*.